Gas discharge lamps are used in a wide variety of applications and environments including direct illumination and display illumination. An example of display illumination is backlighting for liquid crystal or other pixelated transmissive displays. A liquid crystal display backlight often includes one or more gas discharge lamps, which can be cold cathode fluorescent lamps, hot cathode lamps, or other types of lamps as is known in the art. Backlight illumination from the gas discharge lamp is diffusively transmitted through the display panel to a user or observer.
In many consumer applications, such as laptop computers, the illumination brightness provided by a gas discharge lamp varies over a relatively narrow range. The brightness range in such applications can have a dimming ratio of about 30:1, which represents the ratio between the maximum and minimum illumination brightnesses. In contrast, some industrial and military applications can require dimming ratios of up to about 20,000:1. An exemplary application in which such high dimming ratios are desirable is liquid crystal displays used for aviation control instruments, particularly for military aircraft. Control instruments in a military aircraft must be viewable over a wide range of lighting conditions ranging from extremely bright sunlight requiring a display output of about 200 foot-lamberts to pitch darkness in which a display brightness of 0.01 foot-lambert or less is desirable.
Current gas-discharge lamp energizing circuits include a voltage transformer and an inductor (ballast) connected in series with the lamp. The operation of such energizing circuits may be described by considering the transformer voltage to be constant after energizing the lamp, as is typical, and recalling that the voltage drop across an inductor is dependent on the time derivative of the current through the inductor. When the lamp is energized (i.e., at turn-on) there is no voltage drop across the inductor until a breakdown or start-up voltage occurs across the lamp.
The breakdown voltage ionizes gases within the lamp and it conducts current, which causes a voltage drop across the inductor (ballast). Gas ionization dynamics in the lamp cause the voltage across it to decrease as the current through it increases. After the arc start-up, the inductor (ballast) automatically provides a constant voltage and, for a given light output, a constant current, to maintain a controlled low-power arc. Ballasts usually have magnetic cores to reduce their size, and sometimes the ballast functions are incorporated into the transformer.
In typical energizing circuits, the ballast provides a preset lamp operating voltage and dimming is achieved by limiting current. For example, dimming methods employ current-restricting waveform-modulating techniques, such as pulse-width modulation or pulse-train gating, to limit the energizing power delivered to the lamp.
Limited dimming ratios in typical gas discharge lamps arise from the inability of conventional ballasts to maintain a perceptively constant arc at low illumination levels. At lower illumination levels, the arc in a gas discharge lamp driven by a conventional ballast undergoes perceivable interruptions that cause the lamp to appear to "flicker." A lamp is considered not to be in normal operation when it is flickering. High illumination levels are instead limited by physical capabilities of the lamp.
In accordance with the present invention a gas discharge lamp controller controls and drives a gas discharge lamp, which may be either a hot cathode lamp or a cold cathode lamp, whether fluorescent lamp or not. The lamp controller separately varies the current and the voltage that are delivered to the lamp to drive it in an illuminated state over a wide range of brightnesses, including very low brightness levels without flicker. The range of brightnesses is sometimes referred to as the dimming ratio, which is the ratio between the brightest and dimmest illuminated states of the lamp. In one implementation, for example, the lamp controller circuit can provide a lamp with a dimming ratio of over 90,000:1.
The lamp controller includes a brightness control circuit and a driver circuit. In one implementation, the brightness control circuit includes a user-manipulated analog dimmer control, such as variable control voltage (e.g., 0-5 volts), for controlling illumination brightness. For example, a user can control illumination brightness by manipulating a control (e.g., a potentiometer) that selects a magnitude for the variable control voltage. The magnitude of the analog control voltage represents a brightness to be formed by the lamp. An analog-to-digital converter receives the control voltage and generates a digital control value corresponding to the control voltage magnitude.
The digital control value is delivered to a microprocessor or a microcontroller that generates arc current and arc voltage control signals for generating the illumination brightness selected by the user. Signal values corresponding to the arc current and arc voltage control signals are stored, for example, in a memory circuit coupled to the microcontroller. Digital to analog converters convert the arc voltage and arc current control values to analog control signals that are delivered to alamp driver circuit. In one implementation, the lamp driver circuit generates lamp drive signals in the form of current pulses corresponding to the selected brightness. The current pulses for a range of brightnesses may have a fixed pulse period or any of a range of pulse periods that are separately selectable according to the brightness that is selected. While this implementation employs a programmed controller (e.g., a microcontroller) and other digital circuitry, it will be appreciated that the arc current and arc voltage could in the alternative be controlled separately by digital circuitry without a programmed controller or by analog circuitry.
An aspect of the present invention is the determination that flickering at lower illumination levels can be prevented by increasing the voltage of the arc drive signal at low brightness illumination levels. In contrast, current gas-discharge lamp energizing circuits apply a drive signal of a fixed voltage and achieve lamp dimming by varying the arc current alone. Separate and independent control of the arc current and arc voltage allow the arc voltage to be increased at the relatively low arc currents associated with low brightness illumination. Stable low brightness illumination allows the dimming ratio of even conventional lamps to be extended dramatically, thereby providing illumination ranges that are suitable to a wide variety of background (e.g., environmental) lighting conditions.
Additional objects and advantages of the present invention will be apparent from the detailed description of the preferred embodiment thereof, which proceeds with reference to the accompanying drawings.